Pump lasers are generally found in the form of Fabry Perot (FP) cavity lasers whose multimode spectrums are broadband and extremely sensitive to temperature and laser drive current. Such lasers, therefore, require wavelength stabilization for most applications, such as, for example, with optical amplifiers. Different methods have been proposed in the past to stabilize such pump lasers.
In a first prior art laser stabilization method, a laser source is coupled at its output to a reflection filter that selectively reflects back a part of the output of the laser sources toward the laser to stabilize the laser source's spectrum and power. The reflection filter sets both the wavelength and the amount of reflection used to feed back a signal to the laser source as found in, for example, Fiber Bragg Gratings (FBG) stabilized lasers. In such FBG system, the pump laser is connected to the FBG via a Polarization Maintaining (PM) optical fiber. The FBG provides the required reflection for stabilization of the FP laser chip. This method has been extensively used to stabilize a single laser source. Some multiple wavelength applications have also used this method to stabilize multiple laser sources using individual FBG for each laser source followed by a wavelength Division Multiplexer (WDM) to combine stabilized laser source signals.
Referring now to FIG. 1, there is shown a stabilized laser system 10 illustrating a second prior art laser stabilization method. The system 10 comprises a laser source 11 whose output/input facet 12 is coupled to an input/output port 13 of a laser stabilization system 14 (shown within a dashed line rectangle) comprising a transmission filter 15 and a reflector 16. The transmission filter 15 is coupled at an output/input port 17 thereof to an input/output port 18 of the reflector 16. An output port 19 of the reflector 16 provides an output signal from the stabilized laser system 10. The transmission filter 15 sets the wavelength (hereinafter also designated “w”), and the reflector 16 sets the amount of signal reflection provided back through the transmission filter 15 to the laser source 11.
For the system 10 of FIG. 1, an overall Forward filter spectral response for the transmission filter 15 is defined as Fo(w) as the transmission filter spectral response between the output/input facet 12 of the laser source 11 and the output port 19 from the reflector 16. A Feedback filter spectral response for the transmission filter 15 is defined as Ff(w) as the overall transmission filter spectral response between the forward and backward (feedback) signals found at the output/input facet 12 of the laser source 11. In operation, an output signal of the laser source 11 at its output/input facet 12 is filtered by the transmission filter 15, F(w), to provide a signal fo(w) in the path between output/input port 17 and input/output port 18, and is partially reflected back by the reflector 16. A main portion of the signal received by the reflector at its input/output port 18 is transmitted through the reflector 16 to the output port 19 of the system 10. Therefore, the output signal, Fo(w), at output port 19 of the laser stabilization system 10 is defined as:Fo(w)=fo(w)/fi(w)=F(w)  (1)The reflected or feedback signal, ff(w), from the reflector 16 is filtered by the transmission filter 15 for a second time to generate a signal ff(w) that is fed back into the output/input facet 12 of the laser source 11. Therefore, the Feedback filter spectral response is defined as Ff(w) that is further defined as:Ff(w)=ff(w)/fi(w)=F(w)·F(w)  (2)
Referring now to FIG. 2, there is shown a graph of wavelength on the X-axis versus amplitude on the Y-axis for a curve 20 showing an exemplary forward filter spectral response and a curve 22 (shown as a dashed line) for a feedback filter spectral response obtainable in the laser stabilization system 10 shown in FIG. 1 for the transmission filter 15. If the stabilized laser system 10 uses a transmission filter 15 followed by a broadband reflector 16 in the manner shown in FIG. 1 for a single laser system 10, a red shift (a shift to a longer wavelength) in the signal center wavelength 24 from the laser 12 (shown by curve 22) can cause significant excess loss (filtering loss) depending upon the filter transmission bandwidth and the wavelength shift. As a laser drive current increases, the signal center wavelength shifts from a peak wavelength of the transmission filter F(w) 20 to a longer wavelength and suffers a higher insertion loss as shown by exemplary point 26 on the curve 20.
In operation, the output signal (not shown in FIG. 2) generated by laser source 11 is filtered by the transmission filter 15 having a spectral response curve as is shown by curve 20 to provide a desired output signal from the system 10 where the power peaks at a center wavelength corresponding to the peak of curve 20. When a portion of this filtered signal is reflected by reflector 16, it is again filtered by the transmission filter 15 resulting in the narrower dashed line curve 22 and returned to the input of the laser sources 11. It is found that the laser source, in response to a feedback signal, produces a wavelength shift relative to the center wavelength of feedback signal and now generates an output signal that now has a center wavelength shown by line 24 which is separated by an amount δw from the peak of curve 20 as is shown in FIG. 2. This results in an excess loss of power in the output signal of the system 10 from the desired output signal that has a center wavelength at the peak of curve 20. The above description indicates that the laser source 11 produces a red shift in response to a reflected feedback signal. The occurrence of a red directional shift (in a first direction) shown in FIG. 2 is mostly true for semiconductor diodes lasers. However, there are other types of lasers that actually produce a blue shift (to a shorter wavelength) in response to the reception of a feedback signal that also causes a similar excess loss.
This second laser source stabilization method shown in FIG. 1 is not really effective for a single laser source system, but it can be used effectively in stabilizing a multiple wavelength laser source system (not shown). In such multiple wavelength laser source system, multiple laser signals from a multiple laser source are multiplexed using a transmission filter/multiplexer coupled at an output thereof to a broadband reflector. The spectral response of the combination of the broadband reflector and the transmission filter/multiplexer for the demultiplexed laser signals returning to corresponding lasers of the multiple laser source of the system, stabilizes each laser at a predetermined center wavelength.
In an FBG stabilized laser system, the FBG is a reflection filter and, therefore, the signal outside of the reflection spectral band is not subjected to any additional loss (excess loss or filtering loss). As a result, this red shift does not impose significant limitations on the operation of the FBG stabilized laser if the particular application can tolerate a wavelength shift of up to 1 nanometer depending on both the FBG reflection bandwidth and the laser drive current and power.
It is desirable to provide a more efficient single or multiple laser source system that reduces loss for a single or multiple laser source stabilization system based on the use of a transmission filter of various technologies.